The immune system plays a critical role in pathogen clearance and tumor surveillance. Key cellular players in the immune system include T and B lymphocytes and dendritic cells, which exist in a multitude of differentiative and activation states. Their function in immunity critically depends on their state. There is a large knowledge and reagent base available to evaluate the different developmental and activation states of lymphocytes and dendritic cells, particularly in mice. However, as the knowledge base evolves, we find it necessary to use more sophisticated tools to evaluate complex phenotypes. A powerful tool available today is multi-parameter flow cytometry. In the studies described in this application, flow cytometry is being used not only to define cellular subpopulations, but also to study receptor-ligand binding kinetics, biochemical signaling pathways, cytokine secretion, cell cycle status, and cellular trafficking within the body. This application requests funds to purchase a state-of-the-art instrument for multi-parameter flow cytometry--the LSR II flow cytometer. The accessories requested (4 lasers) give this instrument the ability to collect 10 parameters or "colors" of information on each cell. This instrument will allow us to better understand the complex functional roles of subsets of lymphocytes and dendritic cells.